1. Technical Field
The present invention relates to methods of ethanol fermentation. More specifically, the present invention relates to processing stillage.
2. Background Art
Throughout this application, various publications, including United States patents, are referenced by author and year and patents by number. Full citations for the publications are listed below. The disclosures of these publications and patents in their entireties are hereby incorporated by reference into this application in order to more fully describe the state of the art to which this invention pertains.
Ethanol fermentation is the biological process by which sugars are converted into ethanol and carbon dioxide through yeast fermentation. Corn is one of the main feedstock materials used to produce ethanol. Dry milling has previously been used to produce ethanol from corn on other starch sources through fermentation (shown generally in FIG. 1, labeled “Prior Art”). Corn is milled to a flour, slurried, and treated with enzymes to convert the starch to sugars. The sugars are converted to ethanol in large fermenters. The ethanol is recovered through a distillation process. The residual spent grain, referred to as whole stillage, contains corn germ, corn bran, corn oil, unconverted starch, unfermented sugars, yeast cells, yeast metabolites, and other suspended and dissolved solids. The whole stillage stream is generally separated into wet distillers grain (WDG) and thin stillage. The wet distillers grains can be dried to produce Dry Distillers Grain (DDG). A portion of the thin stillage, referred to as backset, is recycled to the front end of the ethanol process as make up water. The remaining thin stillage is evaporated to a syrup, added to the wet distillers grains and dried as Dried Distillers Grains with Solubles (DDGS). WDG, DDG, and DDGS are important co-products that are critical to the economic viability of the ethanol process. However, their value can be enhanced by extracting more valuable co-products from these streams. It has only recently been a goal to recover additional materials from the co-products for further use.
Materials, such as oil, protein, and other solubles, in the whole stillage are very valuable; however, recovery has shown to be inefficient and uneconomical. Recently, various methods have been attempted to recover the additional materials from stillage. These methods include traditional separation techniques such as heating the stillage stream and performing evaporation, using centrifugation, or using membrane filtration, in order to recover these additional materials. The result of each of these separation processes on stillage is a concentrate and a water phase wherein most of the solids have been removed.
U.S. Patent Application Publication No. 2009/0250412 and U.S. Pat. No. 7,608,729 to Winsness, et al., disclose methods for recovering oil from stillage including oil resulting from a process used for producing ethanol from corn. Winsness, et al. generally believe that filtration increases operating costs and therefore focus on separation by heating. In one embodiment, the method includes heating the stillage to a temperature sufficient to at least partially separate, or unbind, the oil therefrom. The heating step includes heating to a temperature above 212 degrees F. but less than about 250 degrees F. The method also includes the step of pressurizing the heated stillage to prevent boiling. The method further includes recovering the oil from the stillage using a gravity separation process including centrifugation. While oil can be recovered from this method there are many products in the thin stillage that are not recovered. For example, the process disclosed by Winsness et al. does not include recovery of stickwater and ProFat fractions (as defined below) and alternative uses for stickwater. Furthermore, it is generally accepted in the art that heating the thin stillage to higher than 250 degrees F. is harmful to proteins and other biological components.
U.S. Pat. No. 6,106,673 to Walker discloses a process and system for the separation of a fermentation process byproduct into its constituent components and for the subsequent recovery of those constituent components. The process requires 1) heating of a mixture containing the byproduct so as to separate the oil from a base component of the byproduct to which the oil is bound at a temperature from about 140 degrees F. to about 250 degrees F., followed by 2) recovering the base product, oil, and possibly other substances such as molasses from the mixture. The process can be performed on a large scale and in a continuous fashion using a mechanical separator to recover fibers from a heated mixture to produce a solids stream and a liquor stream and by then removing oil and insoluble substances from the liquor stream in an evaporator assembly. Energy consumption and water consumption are minimized through 1) the use of waste heat from the system's dryer as an energy source for the evaporator assembly and 2) the use of condensed liquids from the evaporator assembly to dilute the mixture. Fibers recovered during the process can be dried efficiently in a way that produces a superior product.
U.S. Pat. No. 5,250,182 to Bento, et al. discloses a step-wise membrane separation process to recover lactic acid and glycerol, together from thin stillage in an ethanol stream. In each step, the permeate recovery is at least 50%. In a first step, an ultrafiltration (UF) membrane means produces a UF permeate stream in which not only essentially all the insoluble portion of the thin stillage greater than about 0.005 μm is removed as UF concentrate, but also at least 50% of solubles having a molecular weight >2×105 Daltons, including dissolved proteins in the thin stillage. In a second step to which the UF permeate is fed, a nanofiltration (NF) membrane produces a NF permeate with a rejection of less than 30% of both the lactic acid and the glycerol, preferably less than 25%. Essentially all molecules larger than lactic acid or glycerol are removed in the NF concentrate. In a third step, to which the NF permeate is fed, a reverse osmosis (RO) membrane means produces demineralized RO water which contains essentially no lactic acid and glycerol, because these are rejected in the RO concentrate. Use of the membrane separation process in the production of ethanol based on the dry-milling of corn, eliminates the use of a conventional evaporator.
While heating and filtration described in prior art provides some separation of co-products, recovery is limited and costs remain high. One advantage of the present invention is that hydrothermal fractionation of stillage produces a physicochemical alteration, which enables a facile separation allowing for improved recovery of co-products. With respect to the present invention, “hydrothermal fractionation” means heating a substantially aqueous stillage stream to a temperature within a prescribed temperature range, and holding at temperature for a period of time within a prescribed residence time range. A saturation pressure is established and maintained during the hydrothermal fractionation step. Physicochemical alteration means that both physical and chemical changes are imparted to the stillage by the hydrothermal fractionation step. Manifest physical changes include changes in the rate of phase separation, live phase volumetric fractions and phase densities, phase hydrophobicity and changes in color or appearance. Chemical changes include changes in the distribution of protein, fat (oil) and carbohydrate (fiber) components between the substantially liquid phase and the substantially solids phase. These physical and chemical changes are mutually dependent and hence the term physicochemical is applied.
By analogy, thermal hydrolysis has been investigated as a pretreatment step prior to anaerobic digestion of biomass, in particular the anaerobic digestion of waste activated sludge from biological waste water treatment facilities and the pretreatment of cellulosic biomass prior to enzymatic hydrolysis to liberate cellulosic sugars. The former has been commercially implemented while the latter remains a research and development endeavor. Camacho et al. (Combined experiences of thermal hydrolysis and anaerobic digestion—latest thinking on thermal hydrolysis of secondary sludge only, for optimum dewatering and digestion. Proceedings of the WEFTEC® 2008 Conference, Chicago, Ill. Water Environment Federation) reviewed the use of thermal hydrolysis as a pretreatment to anaerobic digestion of activated sludge and noted the improvements in both sludge dewaterability and biogas yield during anaerobic digestion. Optimal treatment temperatures were generally in the range of 150-200° C. (302-392° F.).
Yu et al. (Some Recent Advances in Hydrolysis of Biomass in Hot-Compressed Water and Its Comparisons with Other Hydrolysis Methods, Energy & Fuels 2008, 22, 46-60) reviewed the use of hot compressed water (HCW) as a pretreatment for biomass in the production of cellulosic biofuels. The authors focused on the unique physicochemical properties of HCW and the chemistries imparted by HCW as well as the yield of fermentable sugars resulting from enzymatic hydrolysis of the pretreated biomass.
Kim et al. (including Ladisch) (Enzyme hydrolysis and ethanol fermentation of liquid hot water and AFEX pretreated distillers' grains at high-solids loadings, Bioresource Technology 99 (2008) 5206-5215.) investigated the thermal hydrolysis of dry grind ethanol DDGS as a cellulosic pretreatment prior to enzymatic hydrolysis of the cellulosic biomass. The objective of the thermal treatment of Kim et al. was to prepare the cellulose of DDGS for downstream enzymatic hydrolysis to glucose by cellulase and beta-glucosidase enzymes. U.S. Pat. No. 5,846,787 to Ladisch et al. claims use of thermal hydrolysis in the range of 160-220° C. (320-428° F.) as a pretreatment for cellulosic biomass prior to enzymatic treatment with cellulase. To our knowledge, the hydrothermal fractionation treatment of the present invention has not been described in patents or literature as a means to thermally fractionate stillage and produce enhanced stickwater and ProFat fractions thereof in conventional dry-grind corn ethanol plants.
While heating has also been performed as described in the prior art for recovery of corn oil, the processes that use heating have not used a temperature range which causes this physicochemical alteration. The hydrothermal fractionation and induced physicochemical alteration also results in a set of products obtained from the stillage that are not obtainable with the processes described above, providing an economic advantage.
Prior art processes have tried to remove different solids from thin stillage with: a) anionic polymer additives to increase coagulation and precipitation (Hughes, U.S. Pat. No. 8,067,193), b) the use of microfiltration and subsequent ultrafiltration (Prevost, et. al., US 2004/0082044 A1), c) treatment with polyacrylamide and electrocoagulation (Griffith, US 2007/0036881 A1), d) multiple solvent extraction and filtration steps to recover organic solvent soluble solids from the thin stillage. Other prior art processes have described removal of solids from the clarified aqueous phase through the use of filters after separation of concentrated thin stillage into a light oil phase and a heavy aqueous phase (Woods, et. al., US 2011/0275845 A1). And other prior art processes describe separating solids from a processed broth through chemical reaction to increase the water solubility of insoluble cellulosic, melaninic, ligninic, or chitinic solids (Verkade, et. al., US 2009/0110772 A1). Other efforts have involved filtration of depleted lignocelluosic fermentation hydrolysate broth (more like whole stillage) to separate undissolved solids from the liquid phase and create a low solids liquid (Hennessey, et. al., US 2012/0178976 A1 and Hennessey, et. al., US 2012/0102823 A1). None of these prior art methods in solid-liquid separation has been shown to have any effect on the production of clarified water to improve the growth of algae, fungi or other microorganisms in the production of biomass, metabolites, bio-products, and/or extracts. In other words, the clarified thin stillage of the prior art has very different properties than the stickwater fraction produced in the present invention, and the stickwater fraction of the present invention is shown to be advantageous as a growth media.
Therefore, there is a need for a method of producing a physicochemical alteration that changes the co-products in the stillage and enables facile separation of co-products from stillage streams in ethanol processing as well as providing streams suitable for improving biological production and recovery of valuable co-products, extracts, metabolites and treated water.